For the biomonitoring of DNA damage in humans potentially exposed to genotoxic agents, and optimal method should be capable of detecting many classes of DNA damage in a variety of cell types, should provide data at the level of the individual cell, and should be sensitive, rapid and cost effective. An alkaline microgel electrophoresis assay has been developed which appears to fulfill these goals. The Single Cell Gel (SCG) assay detects, in individual cells, the presence of single strand breaks and alkali-labile sites. Other advantages of this technique include: (1) data are obtained on the intercellular distribution of DNA damage; (ii) extremely small numbers of cells are required (i.e., a few thousand cells); and (iii) its speed, sensitivity and simplicity. The proposed research will further characterize the applicability of this technique as a biomarker for DNA damage in the somatic cells of humans and extend the methodology for evaluating differences in sensitivity to genotoxic agents among individuals. When fully characterized, this technique will allow the monitoring of human populations based on blood samples obtained by fingerprick. This capability should result in the widespread application of this technique in clinical, environmental and occupational medicine.